Transmission networks serve for the transport of user signals, commonly referred to as tributary signals, in the form of multiplexed transmission signals. A transmission network consists of a number a physically interconnected network elements such as add/drop multiplexers, terminal multiplexers, and cross-connects. The physical interconnection between two network elements is referred to as a section or link while the route a particular tributary takes through the transmission network from end to end is known as a path. A path is represented by a multiplexing unit such as a virtual container (VC-N) with its associated path overhead (POH) in SDH (Synchronous Digital Hierarchy). Conversely, a section is represented by an entire transmission frame such as a synchronous transport module (STM-N) with its associated section overhead (SOH).
A very basic aspect of transmission networks is availability of service. Hence, a transmission network needs to provide the means and facilities to ensure sufficient availability. Typically, these network mechanisms are distinguished in protection and restoration. The principle of both is to redirect traffic of a failed link or path to a spare link or path, respectively. Restoration means network management interaction to determine an alternative route through the network after occurrence of a failure while protection uses dedicated protection resources already available and established in the network before a failure might occur.
In order to restore a failed link or the paths on the link, the management plane needs to locate the failure in the network. This is typically achieved by analyzing alarm reports generated by the various network elements. In particular, various monitoring functions are provided at various network elements along a particular path. In the case of a failure, each of these monitors submits an alarm report. The management plane is thus flooded with a huge number of alarm reports. Fault localization is complex because the manager has to process and correlate all these alarm reports. Moreover, restoration must be delayed because the alarm reports will arrive within a certain time window.
Furthermore, fault localization by means of alarm report analysis is only feasible in centrally managed transmission networks. However, transmission networks are currently being developed where at least some functions residing in the management plane are no longer centralized in a central network management system but will be distributed across the entire network. In such a distributed control plane, a different fault localization mechanism is needed. In a first step, network elements adjacent to the fault location have to detect the failure and update their routing databases accordingly. In a second step, the routing database updates must be propagated throughout the entire network by means of routing protocols, which are running in the control plane of the network. It can take a significant amount of time to propagate the new link state information through the network upon occurrence of a failure. Moreover, this update process is not deterministic.
It is therefore an object of the present invention to provide a method and corresponding network devices which allow simplified and faster fault localization in a transmission network and which can also be employed in a distributed network management plane.